A type of sonar system having particular advantages in modern minehunting and salvage operations is commonly termed the shadowgraph type. In this type sonar system, the sonar is constructed such that the echo producing acostic energy is beamed at a low angle relative to the bottom. This grazing incidence of the ensonifying energy striking an object standing proud of the bottom produces a characteristic shadow which, by virtue of its shape, greatly assists in identifying the object struck by the ensonifying sound beam. Irregularities on the bottom, marine life, and small objects produce a multitude of small shadows and highlights on the indicator scope of such systems. These irregularities, termed clutter, make monitoring the indicator scope for a desired target a fatigueing duty.
On a relatively small ship, such as a minesweeper or salvage vessel, long or frequent watches are necessitated by limited crew complement. The fatigue introduced by long hours of close attention to a clutter-filled indicator scope increases the likelihood that operating personnel will make errors of identification.
If an operator could be freed from focusing his undivided attention on his indicator scope and could be alerted when the indicator scope has a target displayed thereon, the length of the watches could be increased without sacrificing accuracy. The resulting saving of personnel and the increased accuracy afforded by such a system would be a significant advance in the echo ranging arts.
A system having an audio indication denoting when signals of the predetermined desired type are received is a desirable solution to the deficiencies inherent in the prior operational arrangements. Prior attempts to produce such alarms have not been successful because of inabilities to differentiate between random signals attributable to marine life and other operational phenomena and the predetermined signals. Too, prior art attempts have had a limited flexibility and have been unable to cope with varying operational circumstances such as bottom debris and litter.